1. Field of the Invention
The invention relates to a source of electrons and to its manufacturing method.
The invention can be applied to the sector of field effect cathodes and makes it possible to obtain, throughout the surface of the devices in question, an electron emission constituted by parallel beams coming from each micropoint electrode.
2. Discussion of the Background
FIG. 1a shows a schematic diagram of a field effect microcathode. Owing to the small dimensions of the basic structure, it is possible to assemble some 10.sup.6 elements identical to that of FIG. 1a per cm.sup.2 (see FIG. 1b), which may have advantages for electron guns in particular. One of the drawbacks of this type of microcathode lies however in the big aperture of the beam emitted at each point electrode. FIG. 2 gives a schematic view of this situation. Owing to this big aperture at each micropoint electrode, it would appear that it is extremely difficult to be able to focus (see FIG. 3) or process the electron beams emitted from an array of microcathodes such as this, which limits their practical value.
In order to resolve this problem, it has been proposed to add a second gate electrode to the structure of FIG. 1a, this second gate electrode being located above the first one and being carried to a lower potential, so as to make the beam extracted from each micropoint electrode (see FIG. 4) parallel (apart from a few aberrations). In this way, it is possible to envisage the focusing of all the beams emitted by an array of microcathodes, by means of a standard electronic optical system (see FIG. 5).
One of the drawbacks of the structure presented in FIG. 4 is that the second electrode is superimposed on the extraction gate and insulated by a second dielectric D2 which should have substantially a thickness equivalent to that of the gate dielectric D1, given the focusing voltages that are liable to be used. For gate diameters of the order of one micron, it is possible that (owing to the big aperture of the emitted beam) there could be an interception of a non-negligible part of the current emitted at each micropoint electrode both by the dielectric D2 supporting the focusing electrode G2 and by this same focusing electrode. With respect to the dielectric D2, this may lead firstly to problems of the emission of secondary electrons which would be parasitic with respect to the main beam and, secondly, problems of the appearance of localized electrostatic charges capable of locally deforming each emitted microbeam. With respect to the focusing electrode G2, the interception of excessive current could quite simply lead to its destruction. One way of overcoming the problem is, naturally, to position the dielectric D2 and the electrode G2 in a recessed position with respect to the opening of the gate, as shown in FIG. 6.
However, the uniform control of this recess on substantial surface areas (of the order of one to several square centimeters) does not seem to be as easy to obtain, and the present invention provides a different approach to resolving this problem of focusing.